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The mantle is chemically heterogeneous, containing distinct geochemical reservoirs created by billions of years of differentiation, subduction recycling, and incomplete mixing. Mid-ocean ridge basalts (MORB) sample the shallow depleted mantle (DMM) -- residual after continental crust extraction -- characterized by low incompatible-element concentrations and depleted isotopic signatures (high epsilon-Nd, low 87Sr/86Sr). Ocean island basalts (OIB) sample deeper, more enriched mantle sources with diverse isotopic signatures defining several end-member components: HIMU (high-mu, recycled oceanic crust with high U/Pb), EM1 and EM2 (enriched mantle with recycled sediment or subcontinental lithosphere), and FOZO/C (a common deep mantle component). The isotopic variability of mantle-derived magmas is the primary evidence for large-scale chemical heterogeneity and long-term recycling within Earth's mantle.
The geochemistry of mantle-derived rocks provides the primary window into Earth's deep interior. Since we cannot sample the mantle directly (with rare exceptions like xenoliths), we reconstruct its composition from the magmas it produces -- and the trace element and isotopic signatures of those magmas encode the mantle's compositional structure.
The depleted MORB mantle (DMM) is the best-characterized reservoir because mid-ocean ridges are the most voluminous volcanic system on Earth (~20 km3/year). DMM is depleted in incompatible elements by a factor of 2-10 relative to primitive mantle estimates, with epsilon-Nd of +8 to +12 and 87Sr/86Sr of 0.7022-0.7035. This depletion reflects the cumulative extraction of continental crust over 4+ billion years. The remarkable chemical homogeneity of N-MORB globally (within a factor of 2 for most incompatible elements) indicates efficient convective mixing of the upper mantle on ~100 Myr timescales.
Ocean island basalts reveal the mantle's hidden complexity. Isotopic plots (87Sr/86Sr vs epsilon-Nd, 206Pb/204Pb vs 207Pb/204Pb) show that OIB define arrays extending from DMM toward several enriched end-member compositions. HIMU (St. Helena, Mangaia) has extreme radiogenic Pb from recycled oceanic crust with high U/Pb. EM1 (Pitcairn, Walvis Ridge) has low epsilon-Nd and moderate 87Sr/86Sr, possibly from recycled lower continental crust or pelagic sediment. EM2 (Samoa, Society Islands) has the highest 87Sr/86Sr (>0.707), attributed to recycled terrigenous sediment. These end-members represent distinct lithologies subducted into the mantle at various times and later sampled by upwelling plumes.
The convergence of isotopic, trace element, and noble gas evidence points to a mantle that is layered in its heterogeneity if not in its convection. The upper mantle is depleted and relatively homogeneous. The lower mantle contains ancient, less-depleted material (evidenced by primitive noble gas ratios in some OIB) plus accumulated subducted material in various stages of mixing and thermal equilibration. Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle may be compositionally distinct thermochemical piles that anchor deep mantle heterogeneity. Understanding this chemical architecture is essential for models of mantle convection, plate recycling, and the long-term evolution of Earth's heat budget.